Generally, organic semiconductor devices including organic diode devices and organic transistor devices are based on the electrical semi-conductivity that relates to the HOMO (highest occupied molecular orbital) energy level and the LUMO (lowest unoccupied molecular orbital) energy level of organic materials. Examples of the organic diode devices include organic light emitting diodes and organic EL (Electroluminescent) diodes, and examples of the organic transistor devices include organic FETs (Field Effect Transistors), organic TFTs (Thin Film Transistors), organic SITs (Static Induction Transistors), organic top gate SIT, organic triodes, organic grid transistors, organic thyristors, and organic bipolar transistors. In these organic semiconductor devices having the organic layers, the electrical and optical characteristics of the devices are strongly dependent on the thin film structure of the organic layers formed on a substrate. Thus, the development of the thin film having an efficient structure is technically as important as the development of new organic materials.
Hereinafter, the structure and the operation of the organic semiconductor device will be described with reference to an organic EL device. The organic EL device which is generally referred to as ‘organic EL’ is one of the self light-emission display devices. In the organic EL devices, the thin film containing fluorescent organic compounds is positioned between electrodes, cathode and anode. The organic EL devices using low or high molecular weight organic compounds as light-emitting materials have many advantages over the EL devices using inorganic compounds as light-emitting materials. These advantages include a simple fabrication, a low driving voltage, and an easy manufacture of large size display and full color display. In the organic EL devices, electrons and holes are injected into the LUMO and HOMO levels of the fluorescent organic compounds of the thin film from the cathode and anode having the opposite electric potential to the cathode, and the injected electrons and holes are recombined to produce excitons, which emit light (fluorescence or phosphorescence) through losing their activity.
FIG. 1a is a structure diagram for illustrating the conventional organic EL device. As shown in FIG. 1a, in the conventional organic EL device, the organic EL layers 20 are positioned between electrodes 11 and 17. The organic EL layers 20 have a multi-layered structure, in which the interface of each layer in the organic EL layers 20 is clearly distinguished. The representative example of the multi-layered structure includes a hole transporting layer 13, a light emitting layer 14 and an electron transporting layer 15 (see: Tang et al., Appl. Phys. Lett. 1987, 51, 913-915). And at need, the multi-layered structure further comprises a hole injecting layer 12 and/or an electron injecting layer 16. When a voltage is applied to the organic EL layers 20 from the electrodes 11 and 17, the electron-hole are recombined at the light-emitting layer 14 of the organic EL layers 20 to induce the light-emission. After fabricating the organic EL device, in order to prevent the organic EL device from being deteriorated due to the exterior air, the substrate (EL panel) on which the organic EL device has been formed is encapsulated with sealing materials (packaging), and is bonded to a cover member. Then, the connectors (FPC, TAB, etc.) are mounted for connecting the encapsulated EL device to an external driving circuit, which produces a passive or active matrix light-emitting device. Such the multi-layered structure has been adopted in almost all kinds of EL devices. For example, Tang et al., produced the organic EL layers of multi-layered structure by sequentially forming a transparent electrode (anode) having high work function, a hole transporting layer, a light-emitting layer with an electron transporting efficiency and a metal electrode (cathode) having low work function. The organic EL devices adopting the above-mentioned organic EL layers have shown luminance of 1,000 cd/m2 with the applied voltage of 10V or below. That is, the organic EL devices suggested by Tang et al. have a low voltage operation and a high light-emitting efficiency. In the above-mentioned organic EL devices, tris(8-quinolinolate)aluminium complex (Alq3) was used as light-emitting materials and Alq3 is known to a good light-emitting materials having a high light-emitting efficiency and electron transporting efficiency.
Another organic EL device with the multi-layered structure has the three-layered structure where a light-emitting layer is formed between a hole transporting layer and an electron transporting layer (see: Jpn. J. Appl Phys. 27 (1988) L269). And still another organic EL device has a light-emitting layer in which the dye (fluorescence pigment such as coumarine derivatives, DCM1 etc.) is doped, thereby controlling light-emitting color thereof and increasing the light-emitting intensity thereof (see: J. Appl Phys., 65 (1989) 3610). Besides the monomeric low molecular weight EL materials, conjugated polymers such as poly(phenylvinylene) were introduced as the EL materials in 1990 by Burroughes et al. (Burroughes, J. H. Nature 1990. 347. 539-541). Recently, stability, efficiency and durability of the polymer EL materials have been remarkably improved.
The organic EL layers 20 can be formed by various methods. Exemplary methods include dry processes such as a vacuum evaporation and a sputtering, and wet processes such as a spin coating method, a cast method, an ink-jet method, a dipping method, and a printing method. Besides, a roll coating method, an Langmuir-Blodgett method and an ion plating method can also be used. The dry process such as a vacuum evaporation has been generally used to manufacture the multi-layered EL device shown in FIG. 1a by using a low molecular weight compound having a good thermal stability and capable of being sublimated to form a thin film. However the dry process requires a high vacuum environment, the manufacturing conditions should be controlled carefully, and thus the process for fabricating EL devices is complex, resulting in the large manufacturing costs. The wet process comprises the steps of dissolving materials which are used for a hole transporting layer, an electron transporting layer and/or a light-emitting layer and a binder resin with a suitable solvent, forming organic layers by spreading the dissolved solution on the surface of the electrode and then evaporating the solvent, and forming an opposite electrode with a method such as evaporation.
The wet process has the following advantages in comparison with the dry process. (1) The wet process can use materials which are difficult to form the film with the dry processes such as vacuum evaporation etc. and use a high molecular weight organic EL materials as well as to a low molecular weight organic EL materials so that it is possible to form a film variously. (2) It is easy to control a very small amount of dopant, which is difficult in the dry process. (3) The manufacture of large size display can be easily carried out. (4) The organic EL layer of the organic EL device can be easily formed relatively and thus the manufacturing cost of the organic EL device is relatively low. (5) Lights with different wavelength can be emitted from each of light-emitting materials at the same time by using various light-emitting materials, so that for example, white light-emission can be easily carried out. (6) Materials of each layer of a conventional EL device are in amorphous state, however, materials of organic layer of the polymer distribution type EL device are distributed in a binder resin and thus the polymer distribution type EL device has a good thermal stability.
The exemplary methods for forming light-emission area with a wet process were disclosed in Japanese Patent Unexamined Publication No. H03-000790 and Japanese Patent Unexamined Publication No. H03-171590. In Japanese Patent Unexamined Publication No. H03-000790 pellinon derivatives or Alq as light-emitting materials are distributed in poly N-vinylcarbazole(PVK), and in Japanese Patent Unexamined Publication No. H03-171590 Alq and tetraphenyl-bendizine as light-emitting materials are distributed in polycarbonate. In the devices having the above-mentioned light-emission area, light-emitting compounds are distributed uniformly throughout the whole light-emission area. Therefore, it is difficult to balance injection and transportation of holes and electrons. As a result, a recombination possibility is decreased and then it is difficult to get sufficient light-emitting efficiency. At need, the organic EL layers of light-emission area can be formed by using the combination of the wet process and the dry process. However, the device having the organic EL layer produced with this method also has a low light-emitting efficiency and requires a high driving voltage.
As one of the solutions to overcome these disadvantages, the method of doping the organic EL layer with ionic salt can be considered. It was reported that the light-emitting intensity is increased when a bias voltage is applied to a light-emitting electrochemical cell for a given time (electrical annealing). Wherein the light-emitting electrochemical cell is manufactured by doping the mixture of polymer such as PPV and ionic conductive materials such as poly(ethylene-oxide)(PEO) with salts such as LiCF3SO3 (see: Y. Yang and Q. Pei, J. Appl. Phys. vol (81), page 3294, 1997). Further, the similar phenomenon to the electrical annealing observed at the high molecular weight EL device which was doped with ions (see: D. B. Romero et al., Appl Phys. Lett. vol(67) page 1659 1995, A. G. MacDiamid and F. Huang, Synth. Met. vol(102) page 1026, 1999). Also, it was reported that the characteristics of the high molecular weight EL device can be improved by ions which was introduced unexpectedly during the synthesis (see: H. Aziz and G. Xu, J. Phys. Chem. B vol(101) page 4009, 1997).
The effects of the above-mentioned ion doping can be explained with the rearrangement of ions doped in the organic layer by the electric field. Namely, when the voltage is applied to the organic EL layers sandwiched between the electrodes, ions doped in the organic EL layers are stacked at the vicinity of the electrodes to form a space charge, thus the light-emitting efficiency of the EL device is increased and the operation voltage of the EL device is decreased.
Specifically, in case of existing ions in the thin layer of high molecular weight compound, ion polarization is induced by migrating anions toward the anode and the cations toward the cathode in the electrical annealing process. The migration of ions produces space charges at the electrodes vicinity. In case that these space charges are stacked at the electrodes vicinity, a large electric field is produced at the electrodes vicinity. The electric field decreases the energy barrier to inject the charge carriers easily. Therefore, the bipolar recombination is increased and the EL device efficiency is enhanced because the injection possibility of holes and electrons is increased greatly.
Also, It was reported that in the case of the dendrimer EL device, the on-set voltage is decreased by changing the charge injection barrier at the boundary of electrode or the light-emitting layer (see: D. Ma et al., J. Phys. D, vol(35) page 520, 2002). And it was reported that when the electrical annealing is carried out by applying electrical voltage of 20V or more to the single light-emitting layer of the PVK, which is doped with an organic low molecule and ammonium salt(tetra-n-butylammonium tetrafluoroborate, Bu4NBF4), the reaction of PVK+BF4→PVK+BF4++e− generates at the electrodes vicinity so that the charges are easily injected (see: Y. Sakuratani, T. Watanabe, S. Miyata; Thin Solid Films V388 (2001) 256-259; Y. Sakuratani, M. Asai, M. Tokita, S. Miyata; Synthetic Metals V123 (2001) 207-210; S. Miyata, Y. Sakuratani, X. T. Tao; Optical Materials 21 (2002) 99-107). However the maximum luminance of the EL device which was fabricated according to the above mentioned method, is about 1000 cd/m2 (at ˜17 V), which is not sufficient luminance. Also, the above-mentioned EL device has other disadvantages including the electrical damage and the deterioration of the uniformity of the light-emission because the electrical annealing voltage is comparatively high (>20 V). These disadvantages result from the uneven charge injection at the light-emitting surface (pixel area). In detail, in the electrical-annealing process, the EL device was heated by Joule heating produced from the current flowing through the device. The temperature of the center of the pixel area is relatively higher than that of the edges of the pixel area due to the low thermal conductivity between the EL device and the substrate on which the EL device is formed. Therefore, ions at the center of the EL device having relatively high temperature migrate easily toward the electrodes. On the other hands, ions at the edges of the EL device having relatively low temperature do not easily migrate toward the electrodes comparatively. Thus, ions are not uniformly spread over the whole electrodes and the uniform charge injection cannot be achieved. As a result, the uniformity of the light-emitting surface is decreased. If the additional electrical annealing will be performed so as to raise the temperature of the edges of the pixel area, ions at the edges of the EL device may easily migrate toward the electrodes and high light-emitting characteristics may be obtained. However, the additional electrical annealing creates the over flowing of current at the center of the EL device so that the EL device is deteriorated and finally the uniformity of the light-emission of EL device is further decreased.